/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    vector<int> inoder;
public:
    bool isValidBST(TreeNode* root) {
        if(root == nullptr) return true;

        if(!isValidBST(root->left)) return false;

        if(inoder.empty() == true || inoder[inoder.size() - 1] < root->val){
            inoder.push_back(root->val);
        } else {
            return false;
        }

        if(!isValidBST(root->right)) return false;

        return true;
    }
};